A Serial ATA (AT Attachment) fail over switch (sometimes referred to as a “Port Selector”) is a component that may provide two paths to a device (e.g., storage device), of which one is the selected/active port/connection. Upon detection by a standby system that the host or host controller communicating through the active port has failed, the active port is switched such that the standby system can take over and continue to have access to the storage device. This is a common approach taken in high-availability and redundant systems such as airline reservation systems, banking systems, and other mission-critical systems where continual up-time is a requirement.
The Serial ATA fail over switch as originally defined was completely transparent to both the device being accessed and the attached hosts. It was so transparent that its presence could not be directly detected and instead had to be inferred by attempting to cause it to switch and determining if a switch occurred. Furthermore, the fail over switch market requirements were that fail over switches be supportable with existing Serial ATA host controllers, so any new feature added to them must be benign/compatible with the controllers already on the market.